2.8 - has the "slow parent" feature been removed?
Looking at https://developer.blender.org/T55026, it does appear that Slow Parent is no longer available and is unlikely to be reinstated.

As an alternative, you could use a Driver to average between the 'desired' location of an object and its 'previous' location. I've done similar in the past before I knew of the existence of the Slow Parent or where I just wanted more control. As an example, see this answer to Moving objects create wind